Numerous plastics and polymers are known from the state of the art that are filled with a filler, for example, an inert mineral powder. A compounding with filler usually takes place in order to lower the average costs of the product or to impart certain properties to the product.
Numerous processes are also known with which fillers can be mixed into a polymer. Thus, the fillers can be added, for example, into a plastic melt. Furthermore, there is the possibility of a “cold mixing”, in which the plastic and the fillers are mixed with each other at low temperatures and the mixture is subsequently heated and melted.
In addition, there is the possibility of adding the fillers to a pre-warmed, softened polymer material and not melting the mixture until in a further step. Such a method is known, for example from EP 1 401 623. In it, the fillers are mixed into polyolefins such as, e.g., polyethylene or polypropylene in that at first the polymer material is brought into a softened state and subsequently the fillers, in the present instance calcium carbonate, are added. The mixture is subsequently melted and/or compressed.
It is also known from the state of the art that thermoplastic materials produced by polycondensation, so-called polycondensates, in particular polyesters, especially PET, can be compounded with fillers, for example, with calcium carbonate. Such filled polyesters are used, for example, as bottles.
However, in the case of polycondensates, in particular polyesters, the particular properties of this type of plastic are generally to be considered that make a recycling and/or reusing of these plastics tricky and problematic.
It should be noted in this regard by way of explanation that, for example, PET can be present in two different structures, namely, in amorphous or in crystalline or partially crystalline form. Amorphous PET is usually transparent, and crystalline PET is opaque or white. As is the case for all thermoplastics that can occur in amorphous or crystalline form, a crystallinity degree of 100% can also not be achieved with PET. Only a part of the structure of PET is capable of orienting itself, that is, to crystallize. Crystalline and amorphous areas alternate with each other, therefore, it is correct to speak of a partial crystallinity. It is possible with PET to achieve a crystallinity degree of approximately 50%. This means that in this state one half of the molecular chains have oriented themselves to each other, that is, they place themselves parallel adjacent to each other or have wound themselves in a circular manner. Therefore, the interactions, in particular van der Waals forces, between the molecular chains obligatorily become greater in the partially crystalline areas. The chains are therefore drawn reciprocally to each and therefore the intermediate spaces between the molecular chains become smaller.
However, the molecular structure of PET can be destroyed by certain factors.
A first degradation mechanism is brought about by thermal degradation of the molecular chains. Here, the bonds between the individual molecules are destroyed by too great a heating. For this reason an appropriate dwell time and a suitable working temperature is to be observed in order to achieve a qualitatively high-grade product.
A second relevant degradation mechanism is the hydrolytic degradation, i.e., PET is, like other polycondensates, susceptible to water and/or moisture:
The water and/or the moisture come(s) substantially from two sources: on the one hand, PET has a hydroscopic structure, i.e., PET absorbs the moisture. This moisture is embedded in the intermolecular intermediate spaces and remains as so-called inner moisture in the polymer itself or in its interior. The inner moisture of the original polymer is a function of the particular environmental conditions. PET has a certain inner equilibrium moisture in the moderate latitudes of approximately 3000 ppm.
Moreover, additional moisture is present on the outer surface of the polymer or of the polymer flakes (outer moisture) that must also be considered during the working.
If too much moisture is present during the working or during the recycling or the extrusion of PET, regardless of from which source, the polymer chains of the PET are hydrolytically split and the initial products, namely, terephthalic acid and ethylene glycol, are partially re-formed in a chemical reaction. This hydrolytic degradation of the chain length of the molecules results in a strong degradation of viscosity as well as a deterioration of the mechanical properties of the end product and disadvantageous changes to the properties of the plastic. The damage can even be so great that the material can no longer be used for the processing of bottles, sheets, etc.
Polyester is especially sensitive, in particular at an elevated temperature, to moisture, in particular, polymer melts react extremely rapidly at approximately 280° C. with water and PET is degraded within seconds. In order to prepare PET again during recycling, however, energy in the form of heat must necessarily be supplied to the material, in particular during the concluding extrusion.
Therefore, in order to protect the polycondensate from hydrolytic degradation and to maintain the polymer chains, all moisture must be removed to the extent possible from the material prior to the pre-preparation or prior to too great an elevation of the temperature and a suitable adjustment of the temperature and of the dwell time must be observed during the working.
Thus, for example, if moist PET is to be worked or prepared and if this PET is introduced into a cutting compressor, then the attempt must be made by suitable measures to prevent hydrolytic damage to the polyester. In order to obtain a qualitatively appropriate end product, it is therefore necessary when recycling or working sensitive polycondensates such as polyesters to reduce to the extent possible the inner moisture as well as the outer moisture adhering to the surface of the plastic. The hydrolytic degradation is not maintained within limits until by an appropriate drying, especially below 100 ppm.
Various engineering possibilities are available for this. Thus, for example, the attempt can be made to remove the outer moisture adhering to the plastic by placing a vacuum or by elevating the temperature.
However, even other engineering problems must be considered here. Thus, for example, amorphous and also partially crystalline PET tend to adhere when heated, which is also a great problem in practice. This problem can only be solved by constant agitation.
Furthermore, it should be borne in mind that some types of plastic are susceptible to oxidative degradation processes, as a result of which the chain length of the plastic molecules is also reduced, which also can entail disadvantageous changes to the properties of the plastics such as, e.g., to the color, strength, etc. Again, in order to prevent this oxidative degradation, there is the possibility of carrying out the working of such sensitive plastics under the avoidance of air, e.g., under an atmosphere of inert gas.
The efficient and economical preparation of polycondensates or polyesters is therefore extremely problematic, among other things, on account of the numerous degradation processes to be taken into consideration and requires a special carrying out of the processes. All this makes the recycling of polycondensates, especially of polyesters and quite in particular of PET, especially problematic and tricky so that an economical recycling of such plastics did not become possible until the development of special ways of carrying out the processes.
Of course, this also applies to the production of polycondensates and polyesters filled with fillers. However, it must additionally be taken into account in the case of filled polycondensates that large amounts of additional moisture are introduced not only via the polymer but also by the fillers themselves, which moisture has a negative effect on the chain length. Thus, for example, calcium carbonate has a very large specific surface and binds large amounts of moisture, namely, above 1000 ppm at 20° and 60% atmospheric moisture.
It was possible in the past to remove outer and inner moisture somewhat effectively from the polymer material with the familiar processes known from the state of the art; however, even additional further moisture is introduced via the filler in a large amount, which results in problems and the process rapidly becomes uneconomical and the quality of the obtained products, i.e., the filled polymers, drops.
This problem is solved in practice in that the fillers are pre-dried in a separate process before being added. The drying of a powdery filler is, however, quite difficult, complicated and expensive. Due to the above-cited problems concerning the hydrolytic degradation of the polymer, it was, however, an absolute necessity in practice to carry out such a pre-drying since otherwise the polycondensate would be too greatly degraded and the quality of the end product would drop. Moreover, this requires an additional process step first, namely, the pre-drying of the fillers, that lengthens the entire process.
Alternatively, there is the possibility of using coated fillers, that is, fillers that are coated on the surface and whose surface is correspondingly reduced as a result. Such coated fillers bind less water and therefore bring less moisture into the plastic material. However, such coated fillers are also considerably expensive and are complex to produce.